The Science

Through a tree-pulling experiment, the scientists found that tree resistance to failure (uproot or snapping) increased with size (diameter at the breast height, DBH, 1.3 m) and aboveground biomass, AGB) and differed among species.

The Impact

This mechanistic approach allows the comparison of tree vulnerability and resistance to snapping and uprooting across tropical and temperate forests and facilitates the use of current findings in the context of ecosystem models. Higher wind-induced tree mortality observed on plateaus and top of slopes may be explained by different wind speeds and gusts direction (valleys have different aspects and the wind can blow parallel or perpendicular), rather than by differences in soil-related factors that might affect the critical turning moment (M_crit).

Summary

High descending winds generated by convective storms are a frequent and major source of tree mortality disturbance events in the Amazon, affecting forest structure and diversity across a variety of scales, and more frequently observed in western and central portions of the basin. Soil texture in the Central Amazon also varies significantly with elevation along a topographic gradient, with decreasing clay content on plateaus, slopes, and valleys, respectively. In this study, the scientists investigated the critical turning moments (M_crit – rotational force at the moment of tree failure, an indicator of tree stability or wind resistance) of 60 trees, ranging from 19.0 to 41.1 cm in diameter at breast height (DBH) and located in different topographic positions and for different species, using a cable-winch load-cell system. The approach used torque as a measure of tree failure to the point of snapping or uprooting. This approach provides a better understanding of the mechanical forces required to topple trees in tropical forests, and it will inform models of windthrow disturbance. Across the topographic positions, size-controlled variation in M_crit was quantified for cardeiro [Scleronema mincranthum (Ducke) Ducke], mata-matá (Eschweilera spp.), and a random selection of trees from 19 other species. The analysis of M_crit revealed that tree resistance to failure increased with size (DBH and ABG) and differed among species. No effects of topography or failure mode were found for the species either separately or pooled. For the random species, total variance in M_crit explained by tree-size metrics increased from an R2 of 0.49 for DBH alone, to 0.68 when both DBH and stem fresh wood density (SWD) were included in a multiple regression model. This mechanistic approach allows the comparison of tree vulnerability induced by wind damage across ecosystems, and facilitates the use of forest structural information in ecosystem models that include variable resistance of trees to mortality inducing factors. Project results indicate that observed topographic differences in windthrow vulnerability are likely due to elevational differences in wind velocities, rather than by differences in soil-related factors that might effect M_crit.

Principal Investigator

G.H.P.M. Ribeiro
National Institute for Amazonian Research (INPA)
gabrielgiga@gmail.com

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

Robinson Negrón-Juárez was supported by the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science under Contract No. DE-AC02-05CH11231, as part of Next-Generation Ecosystem Experiments (NGEE)–Tropics and the Regional and Global Climate Modeling (RGCM) program.

References